TFT switch and method for manufacturing the same

ABSTRACT

A thin-film transistor (TFT) switch includes a gate, a drain, a source, a semiconductor layer, and a fourth electrode. The drain is connected to a first signal. The gate is connected to a control signal to control the switch on or off. The source outputs the first signal when the switch turns on. The fourth electrode and the gate are respectively located at two sides of the semiconductor layer. The fourth electrode is conductive and is selectively coupled to different voltage levels, thereby reducing leakage current in a channel to improve switch characteristic when the switch turns off.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of co-pending patent application Ser.No. 14/119,189, “TFT Switch and Method for Manufacturing the Same”,filed on Nov. 20, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD)technology field, and more particularly, to a thin film transistor (TFT)switch and a method for manufacturing the same.

2. The Related Arts

In detection of an LCD panel, there are two common designs for detectingcircuits. One is that a detecting circuit is directly coupled toscanning lines or data lines in a display area of the panel, and itneeds to cut a connection between a test circuit and a circuit in thedisplay area by using laser after the detection. The other is that thetest circuit is coupled to a signal line in the display area via a TFTswitch. A signal is fed to the display area through the TFT switch whena high voltage level is applied to a gate of the TFT switch to turn iton. On the contrary, since the TFT switch turns off for disconnectionbetween the test line and the scanning or data line in response to a lowvoltage level applied on the gate of the TFT switch when the paneloperates normally, a process of laser cutting is therefore omitted.

Conventionally, there are three electrodes of the TFT switch fordetecting circuits, i.e. a gate, a drain and a source. The gate of theTFT is in negative bias in a long term when the panel works normally,and therefore electrical characteristic of the semiconductor layer ofthe TFT varies to increase leakage current. FIG. 1 shows a circuitdiagram of a conventional switch TFT. As shown in FIG. 1, a shorting baris connected to drains 11 and 14 of the two TFTs. Sources 12 and 15 ofthe two TFTs are respectively connected to different scanning lines (GL)or data tines (DL). When two gates 13 and 16 of the TFTs are appliedwith a low voltage, the two TFTs switch off. If a leakage currentpassing through the two TFTs and the shorting bar occurs, a shortcircuit between the different scanning lines or data lines, i.e. a shortcircuit between different signals, happens and deteriorates displayquality.

SUMMARY OF THE INVENTION

The present invention provides a TFT switch and the method ofmanufacturing the same for reducing leakage current in a channel of theTFT switch to improve switch characteristic when the switch turns off.

According to the present invention, a thin film transistor (TFT) switchcomprises a gate, a drain, a source, a semiconductor layer, and a fourthelectrode. The drain is connected to a first signal. The gate isconnected to a control signal to control the switch on or off. Thesource outputs the first signal when the switch turns on. The fourthelectrode and the gate are respectively located at two sides of thesemiconductor layer. The fourth electrode is conductive and isselectively connected to different voltage levels.

In one aspect of the present invention, the gate and the fourthelectrode are connected to a high voltage level when the switch turnson.

In another aspect of the present invention, the voltage level of thefourth electrode is identical to that of the gate.

In another aspect of the present invention, the voltage level of thefourth electrode is different from that of the gate.

In another aspect of the present invention, the TFT switch furthercomprises a passivation layer and a gate insulating layer between thegate and the semiconductor layer. The drain and the source are locatedbetween the semiconductor layer and the passivation layer. The fourthelectrode is located on the passivation layer. When the switch turnsoff, the gate is connected to a low voltage level. The fourth electrodeis connected to a high voltage level to conduct accumulated electronsaway from the gate side in the semiconductor layer and afterwardsconnected to the low voltage level.

In another aspect of the present invention, a voltage level of thefourth electrode is identical to that of the gate when the fourthelectrode is connected to the low voltage level.

In still another aspect of the present invention, a voltage level of thefourth electrode is different to that of the gate when the fourthelectrode is connected to the low voltage level.

In still another aspect of the present invention, the TFT switch furthercomprises a passivation layer and a gate insulating layer on the gate.The drain and the source are located between the gate insulating layerand the semiconductor layer. The fourth electrode is located on thesemiconductor layer. The passivation layer surrounds the fourthelectrode. When the switch turns off, the gate is connected to a lowvoltage level, and the fourth electrode is grounded to conductaccumulated electrons away from the gate side in the semiconductorlayer.

In yet another aspect of the present invention, the first signal is atest signal, and the source is connected to a scanning or data lineunder test.

According to the present invention, a method for manufacturing a TFTswitch comprises forming a gate connected to a control signal to controlthe TFT switch turning on or turning off, and a gate insulating layer ona substrate in order; forming a semiconductor layer on the gateinsulating layer; forming a drain connected to a first signal and asource on the semiconductor layer, respectively, and covering thesemiconductor layer with a passivation layer; and forming a fourthelectrode on the passivation layer, wherein the fourth electrode isselectively connected to different voltage levels.

According to the present invention, a method for manufacturing a TFTswitch comprises: forming a gate connected to a control signal tocontrol the switch on or off, and a gate insulating layer on a substratein order; forming a drain connected to a first signal, and a source onthe gate insulating layer respectively; forming a semiconductor layer onthe drain and the source to contact the gate insulating layer; andforming a fourth electrode on the semiconductor layer and covering andsurrounding the fourth electrode with a passivation layer, wherein thefourth electrode is selectively connected to different voltage levels.

The benefit of the present invention is that in addition to a gate, adrain and a source inherent to a conventional TFT switch, a fourthelectrode is added in the TFT switch. The drain is coupled to a firstsignal, and the gate is coupled to a control signal to control theswitch turning on or off. The first signal is outputted from the sourcewhen the switch turns on. The fourth electrode and the gate arerespectively locate at two sides of a semiconductor layer. The fourthelectrode is conductive and is selectively coupled to different voltagelevels, thereby reducing leakage current in a channel to improve switchcharacteristic when the switch turns off.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit diagram of a conventional switch TFT.

FIG. 2 shows a TFT switch according to a first embodiment of the presentinvention.

FIG. 3 is a cross-sectional view of the TFT switch according to thefirst embodiment of the present invention.

FIG. 4 is a cross-sectional view of a TFT switch according to a secondembodiment of the present invention.

FIG. 5 is a flowchart of a manufacturing method for the TFT switch shownin FIG. 3.

FIG. 6 is a flowchart of a manufacturing method for the TFT switch shownin FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Please refer to FIG. 2. FIG. 2 shows a TFT switch according to a firstembodiment of the present invention. As shown in FIG. 2, a TFT switch 20comprises a gate G, a drain D, a source S, a semiconductor layer 23among the gate G, the drain D and the source S, and a fourth electrodeB. The drain D is connected to a first signal, and the gate G isconnected to a control signal to control the switch 20 turning on orturning off. The first signal is outputted from the source S when theswitch 20 turns on. The fourth electrode B and the gate G arerespectively locate at two sides of the semiconductor layer 23. Thefourth electrode B is selectively connected to different voltage levels.The gate G, the drain D, the source S and the fourth electrode B aremade of a conductive material.

In the present invention, the TFT switch 20 can be applied to differentcircuits like a TFT switch coupled to a pixel electrode, a test circuit,or a layout circuit of an active-matrix LCD. Preferably, the TFT switch20 is for use in the test circuit. At this time, the first signalconnected to the drain D is a test signal, and the drain S is connectedto the circuit under test, which is data line DL or scanning line GL.The following takes a TFT switch applied to the test circuit as anexample.

In the embodiment, the gate G and the fourth electrode B are connectedto a high voltage level when the switch 20 turns on. At this time, thevoltage level of the fourth electrode B is either the same as ordifferent from that of the gate G. It is noted that the differencebetween the two voltage levels is in a limited range. When the switch 20is to turn off, the gate G is connected to low voltage level, and thefourth electrode B is selectively connected to different voltage levelsfor conducting the leakage current far away from the gate G side in thesemiconductor layer 23. And, then, the fourth electrode B is alsoconnected to low voltage level. The voltage level of the fourthelectrode B is either the same as or different from that of the gate G.It is noted that the difference between the two voltage levels is in alimited range.

FIG. 3 is a cross-sectional view of the TFT switch according to thefirst embodiment of the present invention. As illustrated in FIG. 3, theswitch 20 is structured to arrange the gate G on a base 21, a gateinsulating layer 22 on the gate G, and the semiconductor layer 23 on thegate insulating layer 22. Both of the drain D and the source S are setup on the semiconductor layer 23 and separated by a passivation layer24. The passivation layer 24 also covers the whole surface. The fourthelectrode B is disposed over a gap between the drain D and the source Sand extends onto the drain D and the source S. Two higher electronconcentration n+ layers 25 are respectively set up between the drain Dand the semiconductor layer 23 and between the source S and thesemiconductor layer 23. The two n+ layers 25 are each a part of thedrain D or the source S and greatly reduce channel resistance when theswitch 20 turns on.

In the embodiment, the gate G is located at one side of thesemiconductor layer 23 while the drain D and the source S are located atan opposite side of the semiconductor layer 23. The gate G is connectedto a low voltage level and the fourth electrode B is connected to a highvoltage level to conduct accumulated electrons away from the gate G sidein the semiconductor layer 23 for reducing leakage current when theswitch 20 turns off. The low voltage level of the fourth electrode B iseither the same as or different from that of the gate G. It is notedthat the difference between the two voltage levels is in a limitedrange. The situation that a transition from high to low voltage levelapplied to the fourth electrode B is similar to a transition from highto low voltage level applied to the gate G when the switch 20 turns off.Therefore, the electrons gathered at a location away from the gate Gside in the semiconductor layer 23 are conducted off, similar to theelectrons at a location close to the gate G side in the semiconductorlayer 23.

FIG. 4 is a cross-sectional view of a TFT switch according to a secondembodiment of the present invention. As illustrated in FIG. 4, theswitch 30 comprises a gate G on a base 11, a gate insulating layer 32 onthe gate G. A drain D and a source S are set up on the gate insulatinglayer 32 and are separated by a semiconductor layer 33. Thesemiconductor layer 33 also covers the whole drain D and source S. Afourth electrode B is disposed over a gap between the drain D and thesource S and extends onto the drain D and the source S but doe not coverthe whole drain D and source S. The rest of the surface is covered by apassivation layer 34 surrounding the fourth electrode B and there is acertain gap between the passivation layer 34 and the fourth electrode B.Two higher electron concentration n+ layers 35 are respectively set upbetween the drain D and the semiconductor layer 33 and between thesource S and the semiconductor layer 33. The two layers 35 are each apart of the drain D or the source S and greatly reduce channelresistance when the switch 30 turns on. The gate G, the drain D, thesource S and the fourth electrode B are conductive.

In the embodiment, the gate G, the drain D and the source S are locatedat the same side of the semiconductor layer 33, and the fourth electrodeB and the gate G are respectively located at two sides of thesemiconductor layer 33. When the switch 30 turns off, the gate G isconnected to a low voltage level and the fourth electrode B is groundedto conduct accumulated electrons away from the gate G side in thesemiconductor layer 33 for reducing leakage current. When the fourthelectrode B is connected to a low voltage level, electrons are directlyconducted away from the side of the semiconductor layer 33 that isdistant from the gate G, which is close to the fourth electrode B, viathe fourth electrode B because the fourth electrode B directly contactsthe semiconductor layer 33. The voltage level of the fourth electrode Bis either the same as or different from that of the gate G. It is notedthat the difference between the two voltage levels is in a limitedrange.

Please refer to FIG. 5. FIG. 5 is a flowchart of a manufacturing methodfor the TFT switch shown in FIG. 3. As shown in FIG. 5, themanufacturing method for the TFT switch comprises the following steps:

Step S101: forming a gate, for connecting to a control signal to controla switch on or off, and a gate insulating layer on a substrate in order.

Step S102: forming a semiconductor layer on the gate insulating layer.

Step S103: forming a drain, for connecting to a first signal, and asource on the semiconductor layer respectively and covered with apassivation layer. When the TFT switch is used in a test circuit, thefirst signal is a test signal and the drain connects to the testcircuit. The test circuit is either a data line or a scanning line.

Step S104: forming a fourth electrode on the passivation layer, whereinthe fourth electrode is selectively connected to different voltagelevels.

In the embodiment, the gate is located at one side of the semiconductorlayer while the drain and the source are located at an opposite side ofthe semiconductor layer. The fourth electrode and the gate are connectedto high voltages when the switch turns on, and the voltage of the fourthelectrode is either identical to or different from that of the gate. Thegate is connected to a low voltage, and the fourth electrode isconnected to a high voltage to conduct accumulated electrons away fromthe gate side in the semiconductor layer for reducing leakage currentwhen the switch turns off. Afterwards, the fourth electrode is connectedto a low voltage, and the voltage of the fourth electrode is eitheridentical to or different from that of the gate. It is noted that thedifference between the two voltage levels is in a limited range.

Please refer to FIG. 6. FIG. 6 is a flowchart of a manufacturing methodfor the TFT switch shown in FIG. 4. As shown in FIG. 6, themanufacturing method for the TFT switch comprises the following steps:

Step S201: forming a gate, for connecting to a control signal to controla switch on or off, and a gate insulating layer on a substrate in order.

Step S202: forming a drain, for connecting to a first signal, and asource on the gate insulating layer respectively. When the TFT switch isused in a test circuit, the first signal is a test signal, and thesource is used for connecting to the circuit under test. The testcircuit is either a data line or a scanning line.

Step S203: forming a semiconductor layer on the drain and the source andcontacting the gate insulating layer.

Step S204: forming a fourth electrode on the semiconductor layer andcovering the surrounding of the fourth electrode with the passivationlayer, and the fourth electrode is selectively connected to differentvoltage levels.

In the embodiment, the gate, the drain, and the source are located atthe same side of the semiconductor layer. The fourth electrode and thegate are connected to high voltages when the switch turns on, and thevoltage of the fourth electrode is either identical to or different fromthat of the gate. The gate is connected to a low voltage, and the fourthelectrode is grounded to conduct electrons away from the gate side inthe semiconductor layer for reducing leakage current when the switchturns off. The voltage of the fourth electrode is either identical to ordifferent from that of the gate after the switch turns off. It is notedthat the difference between the two voltage levels is in a limitedrange.

In summary, the present invention provides a TFT switch, which comprisesa gate, a drain, a source, and a fourth electrode. The drain isconnected to a first signal, and the gate is connected to a controlsignal to control the switch on or off. The source transmits the firstsignal when the switch turns on. The fourth electrode and the gate arelocated at two sides of the source and the drain. The fourth electrodeis conductive and is selectively connected to different voltage levels,thereby reducing leakage current in a channel to improve switchcharacteristic when the switch turns off.

Those skilled, in the art will readily observe that numerousmodifications and alterations of the device may be made while retainingthe teachings of the invention. Accordingly, the above disclosure shouldbe construed as limited only by the metes and bounds of the appendedclaims.

What is claimed is:
 1. A thin film transistor (TFT) switch, comprising agate, a gate insulation layer formed on the gate, a drain and a sourceformed on the gate insulation layer and spaced from each other by a gaptherebetween, a semiconductor layer formed on the gate insulation layer,the drain and the source, an n+ layer formed between the semiconductorlayer and each of the source and the drain, and a fourth electrodeformed on and in direct contact with the semiconductor layer and locatedabove the gap between the drain and the source, wherein the drain isadapted to connect with a first signal and the gate is adapted toconnect with a control signal to selectively control the TFT switch onor off so that the source outputs the first signal when the switch turnson; wherein the fourth electrode and the gate are respectively locatedat opposite sides of the semiconductor layer; wherein when the switchturns on, the gate and the fourth electrode are connected to highvoltage levels; and wherein the first signal is a test signal and thesource is adapted to connect to a data line under test; wherein the highvoltage level to which the fourth electrode is connected is identical tothe high voltage level to which the gate is connected.
 2. The TFT switchas claimed in claim 1 further comprising a passivation layer formed onthe semiconductor layer and surrounding the fourth electrode in such away that a gap is present between the passivation layer and the fourthelectrode.
 3. The TFT switch as claimed in claim 1, wherein when the TFTswitch turns off, the gate is connected to a low voltage level and thefourth electrode is grounded to drain off electrons that are accumulatedin the semiconductor layer at a location distant from the gate.
 4. Amethod for manufacturing a thin-film transistor (TFT) switch, comprisingthe following steps: forming, in sequence, a gate and a gate insulationlayer on a base, wherein the gate is adapted to connect with a controlsignal for selectively switching the TFT switch on or off; forming adrain and a source on the gate insulation layer; forming a semiconductorlayer on the drain and the source in such a way as to contact the gateinsulating layer, wherein an n+ layer is formed between thesemiconductor layer and each of the drain and the source and wherein thedrain is adapted to connect with a first signal so that when the TFTturns on, the source outputs the first signal; and forming a fourthelectrode on the semiconductor layer such that the fourth electrode isin direct contact with the semiconductor layer and forming a passivationlayer on the semiconductor layer such that the passivation layersurrounds the fourth electrode, wherein the fourth electrode is adaptedto selectively connect with different voltage levels; wherein when theTFT switch turns on, the gate and the fourth electrode are set at highvoltage levels; and wherein the first signal is a test signal and thesource is connected to a data line under test; wherein the high voltagelevel of the fourth electrode is identical to the high voltage level ofthe gate when the TFT switch turns on.
 5. The method as claimed in claim4, wherein when the TFT switch turns off, the gate is connected to a lowvoltage level and the fourth electrode is grounded to drain offelectrons that are accumulated in the semiconductor layer at a locationdistant from the gate.
 6. A thin film transistor (TFT) switch,comprising a gate, a gate insulation layer formed on the gate, a drainand a source formed on the gate insulation layer and spaced from eachother by a gap therebetween, a semiconductor layer formed on the gateinsulation layer, the drain and the source, an n+ layer formed betweenthe semiconductor layer and each of the source and the drain, and afourth electrode formed on and in direct contact with the semiconductorlayer and located above the gap between the drain and the source,wherein the drain is adapted to connect with a first signal and the gateis adapted to connect with a control signal to selectively control theTFT switch on or off so that the source outputs the first signal whenthe switch turns on; wherein the fourth electrode and the gate arerespectively located at opposite sides of the semiconductor layer;wherein when the switch turns on, the gate and the fourth electrode areconnected to high voltage levels; and wherein the first signal is a testsignal and the source is adapted to connect to a data line under test;wherein when the TFT switch turns off, the gate is connected to a lowvoltage level and the fourth electrode is grounded to drain offelectrons that are accumulated in the semiconductor layer at a locationdistant from the gate.
 7. The TFT switch as claimed in claim 6, whereinthe high voltage level to which the fourth electrode is connected isdifferent from the high voltage level to which the gate is connected. 8.The TFT switch as claimed in claim 6 further comprising a passivationlayer formed on the semiconductor layer and surrounding the fourthelectrode in such a way that a gap is present between the passivationlayer and the fourth electrode.